FRESH‐WATER LENS FORMATION IN AN UNCONFINED BARRIER‐ISLAND AQUIFER¹

Abstract

Cone-penetrometer testing and computer modeling were utilized to investigate factors controlling fresh-water lens formation at Grand Isle, Louisiana. Measurements of tip resistance, sleeve friction, and electrical conductivity were recorded with depth to permit classification of sediment type and to determine thickness of the fresh-water lens and transition zone. Cone-penetrometer testing provided virtually continuous determinations of change in sediment type and ground-water salinity at a resolution rarely achieved using conventional drilling and water sampling techniques.Three sand bodies are present, each separated by a clay layer. The fresh-water lens is thinner in the center of the island than on the flanks. Fresh-water lens thickness is limited by a clay layer which prohibits downward movement of significant volumes fresh water. The transition zone from fresh water to salt water varies in thickness, being thinnest near the Gulf of Mexico and thickest where silt and clay interfinger with the upper sand.Both the thickness of the fresh-water lens and the shape of the transition zone differ from that predicted by theoretical models. Calibration of STJTRA, a variable-density solute-transport model, indicates that permeability variations are the dominant control on formation of the fresh-water lens. (KEY TERMS: hydrogeology; barrier island; modelling; fresh-water lens; cone penetrometer.)

Shallow rainwater lenses in deltaic areas with saline seepage

“…So far, research into fresh rainwater lenses in saline groundwater has mainly been focused on so-called Badon Ghijben-Herzberg (BGH) freshwater lenses in elevated areas like sandy dunes along the coast and on small islands that lack an upward groundwater flow (e.g. Badon Ghijben, 1888; Herzberg, 1901;Fetter, 1972;van Dam and Sikkema, 1982;Meinardi, 1983;Underwood et al, 1992;Collins and Easley, 1999;Bakker, 2000). BGH-lenses are generally thick and the depth of the freshwater-saline interface (H ) is mainly controlled by the relative density difference (α) and the phreatic groundwater level (h): H = h/α.…”

Dynamics of rainwater lenses on upward seeping saline groundwater

“…The geometry of the flow domain is similar to that of the dune problem as described by Underwood et al (1992) (their Figure 13), and Maas (2007), whose analytical solution will be compared with our numerical results. In coastal lowlands, the upper, phreatic aquifer is often separated from the deeper aquifer by a confining layer that is significantly less permeable than the aquifers (Collins et al,1999, Ruppel et al, 2000and Vandenbohede et al 2008. If in such a case the land is drained at a level below mean sea level, the gradient of the driving force (pressure and gravitational forces) will be directed upward and the assumption of uniform upward saline seepage is justified.…”

“…It has been studied by Badon Ghyben and Herzberg (Badon-Ghyben, 1888, Herzberg, 1901 and many others, e.g. Bear (1972), Van der Veer (1977), Bruggeman (1999), Collins et al (1999), Fiori et al (2000), Maas (2007) and Kacimov (2008), and can be regarded as an extension of studies of laterally confined, 2-D groundwater flow to a drain or ditch in the absence of density differences (Childs, 1969). Both cases without and with density differences in the flow domain have received considerable attention (Hooghoudt, 1937, Childs, 1969and Fetter,1972.…”

“…This interest may originate from the need to dimension drainage systems, and because sustainable use of fresh water reserves for drinking water supply or irrigated agriculture require that we understand the dynamics of these water resources. The dune problem has relevance for other geographical situations, such as barrier islands (Collins et al, 1999) and atoll islands (Underwood et al, 1992). Of interest also may be the similarity with immiscible flow of oil spills at the phreatic water surface, as affected by capillarity (Van Dijke et al, 1998a).…”

“…So far, research into lens dynamics has mainly been focused on BGH-freshwater lenses (e.g. Badon Ghijben 1889; Herzberg, 1901;Meinardi, 1983;Underwood et al, 1992;Collins and Easley, 1999;Bakker, 2000;Stoeckl and Houben, 2012). Stoeckl and Houben (2012) examined the development and flow dynamics of freshwater lenses by physical experiments on laboratory scale.…”